1. Field of the Invention
The present invention relates to a cathode ray tube display device using a cathode ray tube including an electron gun having a Gm electrode for modulation and a cathode ray tube display method.
2. Description of the Related Art
FIG. 7 is an explanatory enlarged cross-sectional view of a neighborhood of a cathode of an electron gun in a cathode ray tube (hereinafter referred to as Hi-Gm tube) described in Japanese Patent Laid-Open No. 224618/1999. In FIG. 7, reference numerals 7, 6, 5, 3 and 16 denote a cathode, a G1 electrode for drawing an electron from the cathode 7, a G2 electrode for drawing the electron from the cathode 7, a G3 electrode for drawing the electron from the cathode 7 and an electron emissive material provided on a surface of the cathode 7, respectively. Further, reference numeral 4 denotes a Gm electrode for modulation which is disposed between the G2 electrode and the G3 electrode and is capable of modulate an electron current of electrons emitted from the cathode 7. Furthermore, in an ordinary electron gun, provided are electrodes subsequent to the G3 electrode, namely, for example, a G4 electrode, a G5 electrode and a bead glass supporting a constitution as a whole, namely, for example, the electrodes.
An exemplary constitution of the above-described electron gun is that a thickness of the G1 electrode 6: t1=0.08 mm, a thickness of the G2 electrode 5: t2=0.1 mm, a thickness of the G3 electrode: t3=0.5 mm, a thickness of the G3 electrode 3: t3=0.5 mm, a thickness of the Gm electrode 4: tm=0.1 mm and a material for each of these electrodes is stainless steel (SUS303, SUS304 and the like). Further, intervals between adjacent two electrodes (in an above-described order) are L1 0.8 mm, L2=0.13 mm, L3=0.10 mm and L4=0.9 mm, respectively. Furthermore, a diameter of an aperture of each of the G1 electrode 6, the G2 electrode 5 and the Gm electrode 4 is about 0.35 mm and that of the G3 electrode 3 is about 1.3 mm.
By taking the above-described constitution, while it has been necessary to change a voltage of the cathode 7 as much as about 40 V for changing an emission current which is the electron current by 0 xcexcA to 300 xcexcA for a black-and-white display on a screen, it becomes possible to control the emission current by changing that of the Gm electrode 4 by 10 V and to display by a low voltage.
FIG. 8 is a graph showing a potential distribution in the neighborhood of the cathode 7 of the electron gun 20 in the Hi-Gm tube. In the graph, an abscissa axis and an ordinate axis designate a distance (mm) from the cathode 7 and a potential (V), respectively; a curve 17 shows a potential around a rotational axis of symmetry in the neighborhood of the cathode 7. An arrow mark indicated by reference numeral 18 denotes a region in which the Gm electrode 4 exists (also referred to as existence region)and which is disposed in a distance of about 0.5 mm from the cathode 7. To take an example, the G1 electrode 6, the G2 electrode 5, the G3 electrode 3, the Gm electrode, the anode of the Hi-Gm tube are applied by voltages of 0 V, 500 V, 5.5 KV, 80 V and 25 KV, respectively.
The potential of the Gm electrode 4 is set at 80 V and a dashed line in FIG. 8 shows 80 V. A position (also referred to a minimal position) 19 at which a potential is minimal must exist in the region (also referred to as existence region) 18 in which the Gm electrode 4 exists. When the potential of the cathode 7 is lower than the potential of this position 19, the electron passes through the position 19 and then proceeds in a direction of the screen; however, when higher, the electron can not pass through the position 19 so that it does not proceed in the direction of the screen. When the minimal position 19 exists farther than the existence region 18 seen from the cathode 7, an influence of a potential which the Gm electrode 4 generates becomes smaller, that is, a potential change similar to a case that the voltage of the Gm electrode is lowered is generated from the standpoint of the cathode 7. On the other hand, when the minimal position 19 exists nearer to the cathode 7 than the existence region 18, the influence of the potential of the Gm electrode to the electron current becomes larger, that is, the potential change similar to a case that the voltage of the Gm electrode is elevated is generated from the standpoint of the cathode 7.
In the case of the above-described Hi-Gm tube, since a Gm electrode of an electron gun is disposed in a position much closer to a cathode of the electron, say, about 0.5 mm from the cathode in a direction of a screen, than the cathode ray tube using a conventional electron gun so that, when a temperature of the electron gun is increased one by being heated by a heater and another by allowing a bead current to flow into the cathode, a bead glass which supports the cathode 7 and the Gm electrode 4 is subjected to a heat deformation as well as the cathode 7 and the Gm electrode 4 both of which are made of metal are also subjected to a head deformation whereupon an interval between the cathode 7 and the Gm electrode 4 is changed in a minute degree. The thus generated change of the above-described interval continues until the temperature rise of the electron gun 20 is saturated. Owing to such change, a potential in the neighborhood of the Gm electrode 4 changes whereupon the level thereof at which an electron can pass changes.
Therefore, when the interval between the Gm electrode and the cathode is broadened with the temperature of the electron gun 20, a potential change similar to a case that a Gm electrode voltage is lowered is generated so that, when a cathode voltage is constant, a quantity of electrons which pass is decreased. That is, a quantity of electrons which passes through between an anode 2 and the cathode 7 is decreased whereupon a screen of the cathode ray tube becomes dark.
The present invention has been achieved to solve the above-described problems and has an object to provide a cathode ray tube display device and cathode ray tube display method which is capable of stabilizing an emission current which is an electron current thereby producing a stable luminance of the screen even during a period of time from the time when a power supply of the cathode ray tube display device is turned on till the time when a temperature rise of the electron gun 20 is saturated.
A cathode ray tube display device according to the present invention comprises a time-measuring unit for measuring an elapsed time which is a period of time since a voltage was applied to a cathode ray tube, and a Gm electrode voltage control unit for controlling an applied voltage to the above-described Gm electrode such that a change of an interval between the above-described cathode and the Gm electrode is corrected by the above-described elapsed time.
Further, a cathode ray tube display method according to the present invention comprises the steps of:
measuring time by starting measuring an elapsed time which is a period of time from a time of voltage application to a cathode ray tube;
controlling a Gm electrode voltage for changing an applied voltage to the above-described Gm electrode until the above-described elapsed time reaches a preset time at which a temperature rise of the above-described electron gun is saturated; and
fixing the Gm electrode voltage for stopping changing the applied voltage to the above-described Gm electrode after the above-described elapsed time has gone over the preset time.